Periodic wave repeater



. Sepf- 9, 1941- H. A. WHEELER PERIODIC WAVE REPEATER Filed March so',1939 (I7 REPRODUCN 2l IMAGE DEVICE )IIS --o VIDEO- FREQUENCY AMPLIFIERO- -V--O DETECTOR INTERMEDIATE FREQUENCY AMPLIFIER OSCILLATOR MODU LATORRADIO- FREQUENCY AMPLIFIER IL-o FIELD FREQUENCY GENERATOR SYNCH RONIZINGSIGNAL SE PARATOR -o PROGESSORo-l- FIG.I.

, r fs INVENTOR H OLD A. WHEELER BY a M f (n 1 l f ATTORNEY PatentedSept. 9, 1941 2,255,403 PEmonIc WAVE REPEATER Harold A. Wheeler, GreatNeck, N. Y., assignor to Hazeltine Corporation, a corporation of Dela-Wale Application March 30, 1939, Serial No. 264,935

(Cl. Z50-36) Claims.

This invention relates to improvements in periodic wave repeatersadapted to be excited by periodic pulses and, While of general utility,it is particularly useful in any saw-tooth Wave generator for televisionapparatus having relatively long trace and relatively short retraceintervals and adapted to be synchronized by periodic pulses, being ofspecial utility in that type of generator comprising an oscillatorhaving a vacuum tube which is conductive during the trace intervals.

Periodic wave repeaters adapted to be excited by periodic pulses mayhave an undesirable characteristic, due to the fact that undesiredpulses present with the desired pulses tend to affect the operation ofthe repeater and this is particularly ltrue if the undesired pulses arein amplitude comparable with thedesired pulses. Also, the operation ofsuch repeaters may not be entirely satisfactory because undesired pulsesof the amplitude of desired pulses tend to impair the operation of therepeater at particular points in the cycle. For instance, in a saw-toothwave generator having relatively long trace and relatively short retraceintervals, such as is commonly used in television scanning apparatus,the synchronizing of thel saw-tooth oscillator utilized may be affectedIby undesired pulses as well as desired pulses and, in general, this isparticularly true if the vacuum tube utilized in the saw-tooth scanningoscillator is conductive during the trace interval. Also in somecases,rfor instance, in submultiple-frequency generators such as arecommonly used in the timing chain of television transmitting apparatus,it may be desired to effect synchronization by particular ones of thepulses from a source of periodic synchronizing signals, that is, theones of the pulses occurring at predetermined intervals, and in suchcasesy it is particularly desirable that the apparatus be insensitive toall undesired pulses.

Various methods have been utilized in arrangements of the prior art foreliminating one or more of the above-mentioned disadvantages. Forinstance, it has been proposed in one system to limit the amplitude ofthe source of synchronizing signals so that no undesired pulses appeartherein with an amplitude greater than that of desired pulses. Sucharrangements are not sufficient for satisfactory operation of asaw-tooth scanning generator having an oscillator comprising a vacuumtube which is conductivev during trace intervals of the system, becauseeven a Weak undesired pulse occurring during a trace intervalnecessarily affects the operation of the system. Also, arrangements havebeen proposed for a generator adapted to be synchronized by an externalsource of pulses the purpose of It is another object of the invention toprovide.

a saw-tooth wave generator, having relatively long trace and relativelyshort retrace intervals and adapted to be synchronized by periodicpulses, comprising an oscillator having a vacuum tube which isconductive during the trace intervals but which is protected from beingaffected by undesired pulses in the synchronizing source over the majorportion of the period of the oscillator.

It is still another object of the invention to provide a periodic waverepeater, adapted to be excited by periodic pulses, comprising anarrangement by virtue of which no exciting pulses are applied to therepeater except at predetermined intervals during which it is desiredthat ythe repeater be excited by the external source of pulses.

In accordance with the invention, a periodic wave repeater adapted to beexcited by periodic synchronizing pulses comprises a .vacuum tube havinga pulse-responsive input circuit, a diode effectively in series in theinput circuit for coupling the periodic pulses thereto, means forderiving from the vacuum tube a periodic voltage having a value at apredetermined point of each period substantially diierent than that overthe major portion of the period, and means for applying this derivedvoltage as a bias to the diode with such polarity that it is biased inopposition to conductivity to a lesser degree at the predetermined pointoi each period than over the major portion of each period. The periodicpulses are applied to the diode with such polarity as to tend to renderthe diode conductive and the value of the above-mentioned derivedvoltage of substantially different value is such that the diode isrendered conductive if such value occurs simultaneously with one of theexciting pulses. The amplitude of the derived voltage, over the rnajorportion of the period of the oscillator, however, is of such value thatthe diode is maintained nonconductive even in the presence of a pulse oan amplitude comparable with that of the pulses utilized to excite theperiodic wave repeater.

In a preferred embodiment of the invention, the repeater comprises asaw-tooth wave oscillator for generating pulses having relatively longtrace and relatively short retrace intervals and the oscillator isadapted to be synchronized by periodic pulses from a suitable source.Also in a preferred embodiment of the invention, the above-mentionedderived voltage applied as abias to the diode is a saw-tooth voltageused as a regenerative voltage for the oscillator and, in accordancewith one form of this embodiment, the

vacuum tube of the oscillator is conductive during the trace intervals.

For a better understanding of the invention, together with other andfurther objects thereof, reference is had to the followingdescription/taken in connection with the accompanying drawing and itsscope will be pointed out in the appended claims.

Fig. l of the drawing is a circuit diagram, partly schematic, of acomplete television receiver comprising a line-frequency scanninggenerator embodying the invention, while Fig. 2 comprises graphsillustrating certain of the operating characteristics of the system ofFig. 1.

Referring now more particularly to the drawing, the systemillustrated'in Fig. l comprises a` modulated-carrier television signalreceiver of the superheterodyne type including an antenna system I0, Ilconnected to a radio-frequency amplifier I2 to which are connected incascade, in the order named, an oscillator-modulator I3, anintermediate-frequency amplifier I4, a detector I5, a video-frequencyamplifier I6, and an imagereproducing device I`I. A line-frequencygenerator I8 and field-frequency,generator I9 are coupled to the outputcircuit of the detector I through a synchronizing-signal separator andprocessor 20, the line-frequency generator I8 being coupled to ascanning winding 2| of imagereproducing device I1 in a manner to behereinafter fully described, and the outputl circuit of field-frequencygenerator I9 being coupled to a field-scanning winding ofimage-reproducing device II in a similar or other suitable manner. Thestages or units Ill-I1, inclusive, I9, and may all be of conventionalwell-known construction so that detailed illustrations and descriptionsthereof are unnecessary herein.

Referring briefly, however, to the operation of the above-describedsystem as a whole, television signals intercepted by the antenna I0, IIare amplifled in radio-frequency amplifier I2 and coupled to theoscillator-modulator I3 wherein they are converted intointermediate-frequency signals which, in turn, are selectively amplifiedin the intermediate-frequency amplifier I4 and delivered to detector I5.4The modulation components of the signalare derived by the detector I5and the video-frequency components thereof are supplied to thevideo-frequency amplifier I6 wherein they are amplified and from whichthey are supplied in the usual manner to a brightnesscontrol electrodeof the image-reproducing device I'I. The modulation components are alsosupplied to the synchronizing-signal separator and processor 20, fromwhich suitable synchronizing aasaeoe signals are supplied to the controlcircuits of generatcrs i8 and i3. The intensity of the scanning beam ofdevice il is modulated or controlled in yaccordance with thevideo-frequency voltages impressed upon its brightness-control electrodeinthe usual manner. Scanning waves are generated inthe'line-frequencyand field-frequency generators I8 and I9.,respectively, which are synchronized by the output ofsynchronizing-signal separator and processor 20, and these waves areapplied to the scanning elements of thc imagereproducing device I'I toreproduce scanning fields and thereby to deflect the beam in twodirections normal to each otherl so as to trace a rectilinear l5vpattern on the screen and thereby to reconstruct the transmitted image.

The arrangement of the' present invention is illustrated in Fig. 1 ofthe drawing as applied to a particular type of periodic Wave repeater,specifically, a line-frequency scanning generator I8 adapted to beexcited by periodic pulses which forms the subject matter'of applicantscopending application Serial No. 256,033, led February 13, 1939. Similarcircuit elements ofthe linefrequency generator I8 of Fig. 1 and of themodiflcation of Fig. 5 of the above-mentioned copending application haveidentical reference numerals. Generator I8 comprises a high-muscreen-grid vacuum tube 3|' having a pulse-responsive input circuit anda low-mu reversed triode tube 32. The tubes 3I and 32 have outputcircuits coupled with opposite polarity to the load circuit of thesystem including a parallelresonant circuit comprising a shuntcapacitance 38, shown in dotted lines for the reason that it may becomprised wholly or partially of the inherent capacitance ofrthe'circuit, and the load inductance olf scaning coil 2I. Scanning coil2| is coupled into the parallel-resonant circuit by means of -anautotransformer comprising primary windings 39' and 40' andY secondarywinding 40' is used to transform the low inductance of scanning winding2| to the higher inductance required in the output circuit of tube 3|.The effective inductance of the resonant output circuit of tube 3| isthus that of the transformer primary windings 39', 40 as measured withthe scanning winding 2I connected across the secondary winding 40. s

' Tube 32 is essentially a wave form stabilizing device which isdesigned to act as a negativeresistance during the trace interval,increasing the amplitude of the current in the load circuit andcompensating at least partially for the resistance therein. lt-has noeffect during the retrace intervals and particularly does not obstructthe occurrence of a half-cycle of free 0scillation of the load circuitduring the retrace interval. The plate electrode 22 of tube 32,conventionally used as an anode, is here used -as a control electrodeand is subjected to a control voltage which controls a relatively largecurrent in the output circuit of tube 32, which is here the circuitcomprising the grid 23 and cathode 29. Voltage variations in the outputcircuit of tube 32 during trace intervals are less than the variationsof the control voltage applied to its control circuit by the variationsof the load voltage of the system, which are relatively large. This isdue to the fact that, while arelatively small current is required in itsinput circuit, a relatively large saw-tooth control voltage is requiredin the input circuit of tube 32 and this is obtained from the loadcircuit of the system by virtue of the high peak voltage which existsacross the load circuit during the retrace interval. A saw-tooth voltageis obtained for this purpose across a condenser 4| connected in serieswith a resistor 42 directly across the load circuit. The voltage ofcondenser 4I is coupled to the control electrode of tube 32 through acoupling condenser 43 and a leak resistor 64.

A bias voltage is provided for tube `3| by means of a cathode resistor68 by-passed by condenser 6|. A feed-back circuit is also provided fortube 3|, designed to supply pulse and saw-tooth voltages to its inputelectrode and at the same time to neutralize any kickback into thesource of synchronizing signals 20. This feed-back and neutralizingcircuit includes windings 66 and 61 of the ,output transformer,transformer winding 66, resistor 13, and condenser 68 being provided togenerate a saw-tooth feed-back voltage across condenser 68 in the inputcircuit of tube 3|. Periodic synchronizing pulses from source 28 arecoupled into the input circuit of vacuum tube 3l through a diode 88 andprovide current pulses which are developed into voltage pulses by meansvof resistor BI in the input circuit of tube 3|.

Aside from the particular improvement of the circuit of Fig. 1 whichforms the subject matter of the present invention, the operation of thelinefrequency generator I8 is substantially as described in detail inthe above-mentioned copending application and may be briefly stated asfollows: Assuming that the required current of saw- `tooth wave formflows through scanning inductance 2| during the trace interval, avoltage pulse of high amplitude is developed by the rapid change ofcurrent in the load nductance 2| during the retrace interval, in whichboth tubes 3| and 32 are maintained nonconductive, and a relativelysmall pulse voltage-is developed during the trace interval. The desiredcurrent and voltage wave forms associated with the load circuit of thesystem are obtained by developing currents in tubes 3| and 32 which havesaw-tooth wave forms during trace intervals, but which are reduced tozero during retrace intervals. The required energy is thus supplied tothe load circuit only during trace intervals, a half-cycle of freeoscillation taking place in the load circuit during each trace interval.The saw-tooth voltages developed across condensers 4I and 68 are appliedto the input circuits of tubes 32'and 3 I, respectively, to provide therequired saw-tooth current during the trace intervals, and the pulsevoltages reduce their current output to zero during the retraceintervals.

Reference is made to Fig. 2 for an explanation of the operation of thepresent invention in the circuit of Fig. 1. pulses from source 28produce voltage pulses on the cathode of the diode 88, as represented bycurve A of Fig. 2, the initial potential of the cathode of the diodebeing taken as a reference value. It is seen that, as applied to thediode-cathode, these negative pulses tend to render the diodeconductive. Peaks b of curve A represent pulses by which it is desiredto synchronize the linefrequency generator I8, while peaks c representundesired pulses which may, for instance, be caused by transientdisturbances in the system. The derived saw-tooth voltage developedacross condenser 68 by` means of transformer winding 66 and resistor 13,which is a periodic voltage having a substantially different value at apredetermined point of each period than over the major portion of eachperiod, is applied as a bias to the anode of The negative synchronizingdiode 88 with such polarity that diode 80 is biased in oppositlton toconduction to the leastdegree at the point of each period at which it isdesired to synchronize the oscillator by means of pulses b, that is, atthe above-mentioned predetermined point of each period. A suitableunidirectional bias is also applied to diode 88 from the lower part ofresistor 68. This bias and saw-tooth voltage as applied across diode 88,together with the pulses from source 28, are represented by curve D ofFig. 2, representing the voltage of anode relative to cathode. 'I'heamplitude of the portion of the saw-tooth bias voltage of substantiallydierent value is such that the diode 88 is rendered conductive if suchvalue occurs simultaneouslyl with one of the pulses b, while theamplitude of the portion of the saw-tooth bias voltage during the majorportion of the period is such that diode 8,8 is maintained nonconductiveeven in the presence of a pulse c of amplitude comparable to that of thepulses b. Since the diode 88 becomes conductive only when its anode ispositive with reference to its cathode, it is apparent that no pulsesare passed by the diode except at the precise time when its effectivebias, due to the unidirectional-bias voltage developed across resistor68 and the derived saw-tooth bias voltage across condenser 68,`is zero,that is, when it is desired to synchronize the generator I8; that is,the amplitude of the saw-tooth voltage developed across condenser 68 isof such value at the time corresponding to a pulse b as to render thediode 88 conductive, since such value occurs simultaneously with asynchronizing pulse, but its value over the major portion of the periodis such that the diode 88 is maintained nonconductive even in thepresence of a pulse such as c of amplitude comparable to that of thedesired synchronizing pulses b. When the diode 88 becomes conductive,current pulses flow from source`20 through diode 88, condenser 68, andresistor 8|, and the corresponding voltage pulses developed across theimpedance means comprising resistor 8| are injected into the inputcircuit of vacuum tube 3I, that is, the circuit comprising resistor 8|,condenser 68, and winding 61, the voltage pulses having a magnitudesuilcient to synchronize the generator.

It will be seen that, in the embodiment of the invention illustrated,tube 3| is conductive during the trace intervals of the generated Waveand is, therefore, susceptible to disturbances in the source ofsynchronizing signals during the trace interval. The arrangement of thepresent invention is, therefore, designed to provide a means by whichany disturbances in the source 28 of an amplitude comparable with thedesired synchronizing pulses are kept out of the input circuit of tube3| during the major portion of 4its cycle and thereby are prevented fromundesirably affecting the output current of tube 3I. At the same time, afeed-back voltage of saw-tooth wave form is continuously applied to thecontrol grid of tube 3| from condenser 68 for maintaining the properwave form of the output current of tube 3|..

While there has been described What is at present considered to be thepreferred embodiment of this invention, it will be obvious to thoseskilled in the art that various changes and modications may be madetherein without departing from the invention, and it is, therefore,aimed in the appended claims to cover all such changes and modificationsas fall within the true spirit and scope of the invention.

What is claimed is:

i. .el periodic wave repeater adapted to be excited by periodicsynchronizing pulses comprising, a vacuum tube having a pulse-responsiveinput circuit, a diode eectlv'ely in series in said input circuit forcoupling said pulses thereto, means for deriving from said4 tube aperiodic voltage having a substantially different value at apredetermined point of each period than over the major portion of eachperiod, and means for applying said derived voltage as a bias to saiddiode with such polarity that it is biased in opposition to conductionto a lesser degree at said predetermined point of each period than oversaid major portion of each period, said pulses being applied to saiddiode with such polarity as to tend to render said diode conductive, theamplitude of said voltage of substantially different value being suchthat said diode is rendered conductive if such value occurssimultaneously with one of said pulses, and the amplitude oi saidderived voltage during said major portion of said period being such thatsaid diode is maintained nonconductivey even in the presence oi a pulseof amplitude comparable with that ci said pulses.

2.' A periodic wave generator adapted to be eX- cited by periodicsynchronizing pulses comprising, a vacuum-tube oscillator having apulse-l responsive input circuit, a diode effectively in series in saidinput circuit for coupling said pulses thereto to synchronize saidoscillator, means for deriving from said oscillator a periodic voltagehaving a substantially dierent value at a predetermined point of eachperiod than over the major portion of each period, and

means for applying said derived voltage as a bias to said diode withsuch polarity that it is biased in opposition to conduction to a lesserdegree at said predetermined point of each period than over the majorportion of each period, said pulses being applied to said diode withsuch polarity as to tend to render said diode conductive, the amplitudeof said voltage of substantially different value being such that saiddiode is rendered conductive if such value occurs simultaneously withone of said pulses, and the amplitude 'of said voltage over said major,portion of said period being such that said diode is maintainednonconductive even in the presence of a pulse of amplitude comparablewith that of said pulses.

3. A periodic Wave generator adapted to be eX- cited by periodicsynchronizing pulses comprising, a vacuum-tube oscillator having apulseresponsive input circuit and an output circuit, a diode effectivelyin series in said input circuit for coupling said pulses thereto tosynchronize said oscillator, means for deriving from said output circuita periodic voltage having a substantially diierent value at apredetermined point of each period than over the major portion of saidperiod and for applying said voltage to said input circuit for feedbackin said oscillator, and means V for. applying said derived voltage as abias to said diode with such polarity that it is biased in opposition toconduction to a lesser degree at said predetermined point of each periodthan over said major portionv of each period, said pulses being appliedto said diode with such polarity as to tend to render said diodeconductive, said voltage of substantially different value being suchthat saidfdiode is rendered conductive i! such value occurssimultaneously with one of said pulses, and the amplitude of saidvoltage during said major portion o! said period being En adriatica tiveeven in the presence of a pulse of an ampliture comparable with that cisaid pulses.

d. A saw-tooth Wave generator for generating pulses having relativelylong trace and relatively short retrace intervals and adapted to besynchronized by periodic synchronizing pulses comprising, a vacuum-tubeoscillator having a pulsei responsive input circuit and an outputcircuit, a

such that said diode is maintained nonconducdiode edectively in seriesin said input circuit for coupling said pulses thereto, means forderiving`from said output circuit a periodic sawtooth voltage andapplying said voltage to said input circuit for feedbacky in said vacuumtube, and means for applying said derived voltage also as a bias to saiddiode with such polarity that it is biased in opposition to conductionto a substantially lesser degree at one extreme value or" said saw-toothvoltage, said pulses being applied to said diode with such polarity asto tend to render said diode conductive, the amplitude of said oneextreme value voltage being such that said diode is rendered conductiveif such value occurs simultaneously with one of said pulses and theamplitude of said derived voltage over the major portion of said periodbeing such that said diode is maintained nonconductive even in thepresence of a pulse of an amplitude comparable with that of said pulses.

5. A saw-tooth wave generator for generating pulses having relativelylong trace and relatively short retrace intervals and adapted to besynchronized by periodic synchronizing pulses comprising, a vacuum-tubeoscillator having a pulseresponsive input circuit and an output circuit,a diode eiectively in series in said input circuit for coupling saidpulses thereto, means for deriving from said output circuit a periodicsawtooth voltage having an amplitude relatively large with respect tothe amplitude of said periodic pulses, means for applying said derivedvoltage as a bias to said diode with such polarity that it is biased inopposition to conduction to a lesser degree at one extreme value of saidsawtooth voltage, means for supplying to said' diode aunidirectional-bias voltage of such polarity and magnitude as to reducethe effective bias due to said unidirectional bias and said derivedvoltage approximately to zero" at said one extreme value of saidsaw-tooth voltage, said pulses being applied to said diode with suchpolarity as to tend to render said diode conductive.

6. A saw-tooth current wave generator having relatively long trace andrelatively short retrace intervals and adapted to be synchronized byperiodic synchronizing pulses comprising, a vacumn-tube oscillatorhaving a pulse-responsive input circuit, said vacuum tube beingconductive during said trace intervals and, therefore, susceptible toundesired disturbing pulses during said' trace intervals, a diodeeffectively in series in said input circuit for coupling said pulsesthereto, means for deriving from said tube a periodic voltage having asubstantially different value at a predetermined point of each periodthan over the major portion of said period, and means for applying saidderived voltage as a bias to said diode withsuch polarity that it isbiased 1n oppos1tion to conduction thereof to a lesser dederedconductive if suchy value occurs simultaneously with one of said pulses,and the amplitude of said derived voltage over said major portion ofsaid period being such that said diode is maintained nonconductive evenin the presence of an undesired pulse of -amplitude comparable with thatof said pulses.

7. A saw-tooth wave generator adapted to lbe synchronized by periodicsynchronizing pulses comprising, a vacuum-tube oscillator having apulse-responsive input circuit, a diode effectively in series in saidinput circuit for coupling said pulses thereto, a regenerative circuitfor said vacuum tube comprising a condenser in said input circuit and aresistor for deriving across said condenser a saw-tooth voltage, andmeans for applying said derived voltage as a bias to said diode withsuch polarity that it is biased in opposition to conduction to asubstantially lesser degree at one extreme value thereof, said pulsebeing applied to said diode with such polarity as to tend to render saiddiode conductive, the amplitude of said voltage of said one extremevalue being such that said diode is rendered conductive if such valueoccurs simultaneously with one of said pulses, and the amplitude of saidderived voltage over said major portion of said period being such thatsaid diode is maintained nonconductive even in the' presence of a pulseof an amplitude comparable with that of said pulses.

8. A periodic wave generator adapted to be eX- cited by periodic voltagepulses comprising, a vacuum-tube oscillator having a pulse-responsiveinput circuit and an output circuit, a diode eiectively in series insaid input circuit and responsive to said voltage pulses for couplnginto said input circuit corresponding current pulses, impedance means insaid input circuit for developing a pulse voltage in said input circuitin response to said current pulses to synchronize said oscillator, meansfor deriving from said output circuit a periodic voltage having asubstantially different value at a predetermined point at each periodthan over the major portion oi' said period and for applying saidvoltage to said input circuit for regeneration o! said oscillator, andmeans for applying said derived voltage as a bias to said diode withsuch polarity that it is biased in opposition to conduction to a lesserdegree at said predetermined point of each period than over said majorportion of each period, said pulses being applied to said diode withsuch polarity as to tend to render said diode conductive, the amplitudeof said voltage of substantially different value being such that saiddiode is rendered conductive if such value occurs simultaneously withone of said pulses, and the amplitude of said voltage over said majorportion of said period being such that said diode is maintainednonconductive even in the presence of a pulse of amplitude comparablewith that of said pulses.

9. A periodic wave generator adapted to be excited by periodic voltagepulses comprising a vacuum-tube oscillator having a pulse-responsiveinput circuit and an output circuit, a diode eiectively in series insaid input circuit and responsive to said voltage pulses for couplinginto said input circuit corresponding current pulses, a resistor in saidinput circuit for developing a pulse voltage in said input circuit inresponse to said current pulses to synchronize said oscillator, meansfor deriving from said output circuit a periodic voltage having asubstantially different value at a predetermined point at each periodthan over the major portion of said period and for applying said voltageto said input circuit for feedback in said oscillator, and means forapplying said derived voltage as a bias to said diode with such polaritythat it is biased in opposition to conduction to a lesser degree at saidpredetermined point of each period than over said major portion of eachperiod, said pulses being applied to said diode with such polarity as totend to render said diode conductive, the amplitude of said voltage ofsubstantially different value being such that said diode is renderedconductive if such value occurs simultaneously with one of said pulses,and the amplitude of said voltage over said major portion of said periodbeing such that said diode is maintained nonconductive even in thepresence of a pulse of amplitude comparable with that of said pulses. I

10. A saw-tooth wave generator adapted to be synchronized by periodicvoltage pulses comprising, a vacuum-tube oscillator having apulseresponsive input circuit, a diode effectively in series in saidinput circuit and responsive to said voltage pulses for coupling intosaid input circuit corresponding current pulses, a feed-back circuit forsaid vacuum tube comprising a condenser in said input circuit and aresistor for deriving across said condenser a periodic voltage having asubstantially diierent value at a predetermined point of each periodthanover the major portion of said period, a resistor in series with saidfirstmentioned condenser included in said input circuit for developingvoltage pulses in response to said current pulses, and means forapplying said derived voltage as a bias to said diode with such polaritythat it is biased in opposition to conduction to a lesser degree at saidpredetermined point of each period than over said major portion of eachperiod, said pulses being applied to said diode with such polarity as totend to render said diode conductive, the amplitude of said voltage ofsubstantially different value being such that said diode is renderedconductive if such value occurs simultaneously with one of said pulsesand the amplitude of said voltage over said major portion of said periodbeing such that said diode is maintained nonconductive even in thepresence of a pulse of an amplitude comparable with that of said pulses.

HAROLD A. WHEELER.

